1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related communication device, and more particularly, to a method of handling non-access stratum (NAS) messages in a wireless communication system and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3GPP Rel-8 standard and/or the 3GPP Rel-9 standard is developed by the 3rd Generation Partnership Project (3GPP) as a successor of a universal mobile telecommunications system (UMTS), for further enhancing performance of the UMTS to satisfy users' increasing needs. The LTE system includes a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) for communicating with a plurality of user equipments (UEs) and communicates with a core network including a mobility management entity (MME), a serving gateway, etc. for NAS (Non Access Stratum) control. In UMTS system, a Serving GPRS Support Node (SGSN) is responsible for the delivery of data packets to the mobile devices back and forth within its geographical service area, including packet routing and transfer, mobility management (attach/detach and location management), logical link management, and authentication and charging functions. The MME provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN.
An access point name (APN) is used in the LTE and LTE-advanced (LTE-A) systems for establishing packet data connections, and is managed by the MME. Further, the APN is used for identifying a packet data network (PDN) or a service type (e.g. multimedia messaging service (MMS)), wherein the PDN or the service type is carried on the packet data connections.
The core network manages the mobile reachability by using mobile reachable timer, paging proceed factor (PPF), and implicit detach timer. The core network supervises the periodic tracking/routing/location area updating procedure of the UE via the mobile reachable timer. By default, the mobile reachable timer is set slightly larger than the periodic TAU/RAU/LAU timer. Upon expiry of the mobile reachable timer, the network shall clear the PPF flag and start the implicit detach timer. The value of the implicit detach timer is network dependent. Due to the clear of the PPF, the core network cannot page the UE during the period of the implicit detach timer. If the implicit detach timer expires before the UE contacts the network, the network shall implicitly detach the UE. The implicit detach timer shall be stopped when a NAS signaling connection is established for the UE.
NAS level congestion control contains functions of “APN based congestion control” and “General NAS level Mobility Management control”. The use of the APN based congestion control is for avoiding and handling of EMM and ESM signalling congestion associated with UEs with a particular APN. With General NAS level Mobility Management control, the MME may also use the reject of NAS level Mobility Management signaling requests under general congestion conditions.
When the NAS level mobility management congestion control is applied due to network overload or congestion, the MME rejects an attach request, a tracking area update (TAU), a routing area update (RAU) or a service request from the UE, and the MME replies the UE with a mobility management back-off timer (MMBK) in a reject message. Then, the MME/SGSN enforces the MMBK timer by immediately rejecting any subsequent request from the UE targeting at connecting to the APN before the MMBK timer is expired.
When the UE is configured for dual priority, the UE is configured for NAS signaling low priority and also configured to override the NAS signaling low priority indicator. For example, the UE is allowed to initiate a NAS mobility management (MM) request message without indicating low access priority while an MMBK timer for another NAS MM request message with a low priority indicator is running. However, if the UE is switched off and then switched on before the expiry of an MMBK timer, the UE cannot determine which priority level the MMBK timer is associated to. In such a situation, the performance of the UE may be decreased.
Similar problems occur when the UE configured for dual priority has received a session management back-off (SMBK) timer for a specific APN.
Thus, how to handle NAS messages and back-off timers properly when the UE is configured for dual priority is a topic to be addressed and discussed in the industry.